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   <title>Basic Concepts of Jet</title>
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<font face="Arial Alternative"><font color="#3333FF">Jet:&nbsp; Basic Concepts</font></font></h2>
Jet is based on the concepts of annotation, annotated documents, and annotators
as developed for the Tipster Architecture.&nbsp; An <i>annotation</i> is
a piece of information about a portion of a document.&nbsp; It consists
of
<ul>
<li>
a pointer to a portion of a document (a start and end position)</li>

<li>
the <i>type</i> of the annotation</li>

<li>
a set of <i>features</i> of the annotation, and their values</li>
</ul>
For example, in "My mouse eats cheese.", to indicate that "My mouse" is
a noun phrase, we would record an annotation with start = 0 and end = 10
(end points to the position past the end of the annotation, and includes
the white space following the token), type = "constit" (constituent), and
feature "cat" (category) with value "np".
<p>A <i>document</i> consists of some text (a character string) and some
annotations on that text.&nbsp; In processing the document, we normally
do not alter the text;&nbsp; rather, we add annotations to it.
<p>An <i>annotator</i> is a procedure (method) which analyzes a document,
including the annotations already on it, and adds new annotations to the
document.&nbsp; (It can also delete annotations, although that is less
common.)&nbsp; The processing of a document is organized as a series of
annotation steps, each performed by one annotator.&nbsp; For example, processing
may begin with a text zoner (which finds the portion of the document containing
the text to be further analyzed), a sentence splitter (which divides that
portion into sentences), and a part-of-speech tagger.
<p>An <i>external document </i>is a document which exists outside of a
Jet system, typically as a file or a Web page.&nbsp; When an external document
is <i>opened</i> (converted to an (internal) document), some of the information
from the external document may be converted to annotations.&nbsp; Several
different formats of external documents are recognized.&nbsp; The most
common uses XML-like tags, such as "<tt>&lt;constit cat="np"> ... &lt;/constit>"</tt>
to indicate that some words form an NP constituent.&nbsp; Another format,
for part-of-speech information only, represents data as <i>token</i>/<i>POStag</i>
(for example, "<tt>The/DT cat/NN chases/VBZ mice/NNS</tt>").
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